One way I guide my drilling is with a bullseye (round) bubble level glued on the back end of the hole-hawg or other drill. Gets my body position, hand grip, and 'eyeball' plumb before starting, and helps check plumb-ness while drilling.

The longer the bit (and extension?) the easier it is to be accurate.

A hard knot will try to deflect any bit or boring machine in my experience, at least somewhat. I even built a custom drilling tower as Timbeal suggests, clamped it to the timbers etc, but there is so much torque involved that it would twist and distort in bad knots no matter what.

If perfect mortise walls are an issue, one can drill a little undersize then pare with a chisel.


Pete